1. Field of the Invention
This invention relates to an electrophotographic light-transmitting recording material on which toner images are formed, and a heat fixing method by which the toner images are fixed on the recording material.
2. Related Background Art
In a typicalfull-color toner image forming method conventionally available, a photosensitive material of a photosensitive drum is electrostatically uniformly charged by a primary corona assembly, and is imagewise exposed using laser light modulated by magenta image signals of an original, to form an electrostatic latent image on the photosensitive drum. The electrostatic latent image is developed by means of a magenta developing assembly to form a magenta toner image on the photosensitive drum. Then, to a recording material transported there, the magenta toner image formed on the photosensitive drum is transferred by means of a transfer corona assembly.
Next, the photosensitive drum on and from which the electrostatic image has been developed and has been transferred is destaticized by means of a charge eliminating charging assembly, and is then cleaned. Thereafter, it is again electrostatically charged by the primary corona assembly, and a cyan toner image is similarly formed on the photosensitive drum. The cyan toner image is transferred to the recording material on which the magenta toner image has been transferred, and then a yellow toner image and a black toner image are further successively formed and developed so that the four color toner images are transferred to the recording material. The recording material having the four color toner images formed thereon is fed to a fixing means such as a fixing roller so that they are fixed to the recording material by the action of heat and pressure. Thus, a full-color image is formed.
In recent years, such image forming apparatus are not only used as copying machines for office work to make copies of originals, but also has begun to be used in the field of printers serving as outputs of computers and in the field of personal copying in private use. In addition to such field as typified by laser beam printers, the apparatus are also being applied in plain-paper facsimile machines employing a basic image forming apparatus.
Under such circumstances, the image forming apparatus as described above are sought to be made smaller, lighter, faster, of better quality and more reliable. Such machines have now been constituted of simpler components in various respects. As the result, a higher performance has become required for toners, and superior machines can no longer be accomplished unless an improvement in the performance of toners is achieved.
In recent years, the need for various modes of copying is accompanied with a rapid increase in demand for color copying. In order to more faithfully copy original color images, it is sought to achieve a much higher image quality and a much higher resolution. From such viewpoints, the toners used in the color image forming method are required to have good melt properties and color-mixing properties when heat is applied, and also to have a low melting point and high sharp-melt properties. Use of such toners having high sharp-melt properties makes it possible to broaden the range of color reproduction of copied images and obtain color copies faithful to original images.
Such a toner having high sharp-melt properties, however, has so high an affinity for the fixing roller that it tends to offset to the fixing roller during fixing. In particular, in the case of a fixing means in full-color image forming apparatus, an increase in toner layer thickness more tends to cause the offset since a plurality of toner layers corresponding to magenta toner, cyan toner, yellow toner and black toner are formed on the recording material.
In order to improve the releasability of toner from the surface of the fixing roller, a measure has been conventionally taken such that the roller surface is formed of a material such as silicon rubber or a fluorine resin, having an excellent releasability to toner, and, in order to prevent offset and to prevent fatigue of the roller surface, its surface is further covered with a thin film formed using a fluid having a high releasability as exemplified by silicone oil or fluorine oil. However, this method, though effective in view of the prevention of the offset of toner, requires a device for feeding an anti-offset fluid, and hence has the problem that the fixing assembly becomes complicated. Also, this application of oil brings about the problem that it causes separation of layers constituting the fixing roller to consequently acceleratedly shorten the lifetime of the fixing roller.
The need in recent years for various modes of copying is also accompanied by the use of paper of various types, coated paper, plastic films and so forth as recording materials. In particular, need for light-transmitting sheets (OHP sheets) has attracted notice, which make use of an overhead projector (OHP) for its presentation. Especially in the case of the OHP sheets, as different from paper, the oil used in the above fixing assembly may adhere to the surface of the recording material because of their low oil absorption capacity. As the result, the OHP sheets on which images have been formed can not avoid having a sticky feeling because of the oil coated thereon, to cause a lowering of image quality. Also, the release oil such as silicone oil may evaporate by heat to contaminate the interior of image forming apparatus, and also has a possibility of causing the problem of disposal of recovered oil.
Accordingly, much hope has been put in the establishment of a fixing system that requires no application of oil in the course of fixing and the development of a novel toner for achieving its establishment, having solved the above problems.
To cope with the above subject, Japanese Patent Application Laid-open No. 61-273554 discloses a toner containing a release agent such as wax. The toner containing the wax brings about an improvement in heat conductivity in toner on account of the wax that melts at a low temperature, so that it enables low-temperature fixing. As a more preferable feature, the wax having melted at the time of fixing acts also as a release agent, and hence it becomes possible to prevent high-temperature offset without applying a separate release agent such as oil to the fixing roller.
When color toner images or full-color toner images are formed on the light-transmitting sheet by using an electrophotographic system having a dry development system and the images formed are projected using an OHP, a phenomenon may occur in which, even though the images on the light-transmitting sheet show a satisfactory color formation, the projected images have a grayish tone as a whole to give a very narrow range of color reproduction. This phenomenon occurs because unfixed toner images formed on a smooth light-transmitting sheet can not be melthed properly by the heating in the course of fixing and remain particulate, to cause scattering of incident light and form a shade on the screen. In particular, in halftone areas or highlight areas having a low image density, the absorption ascribable to a dye or pigment in the toner becomes lower because of a decrease in the number of toner particles, so that a phenomenon may occur in which the color tone to be reproduced is grayish.
On the other hand, in the case when toner images formed on recording materials such as plain paper are viewed, reflected images of light shed on fixed toner images are viewed. Hence, the toner surface remaining more or less particulate may have less influence on image quality. However, in the case when toner images are viewed through transmitted light or projected on screens as in OHPs, light transmission properties may become poor because of scattering of light if any residual shape ascribable to toner particles is distinctive. Accordingly, recording materials used in OHPs are required to be effective for making the toner less particulate after the fixing to improve light transmission properties.
Accordingly, as recording materials for electrophotography, light-transmitting recording materials comprising a transparent base sheet provided thereon with a surface layer formed of a thermoplastic resin such as styrene-acrylic resin or polyester resin are hitherto proposed in variety, from the viewpoint of the improvement in sharpness and improvement in transport performance and blocking resistance that are attributable to an improvement in the fixing performance of toners. For example, Japanese Patent Applications Laid-open No. 1-263085, No. 6-19180, No. 6-19485 and No. 6-332221 disclose such recording materials. Also, as a means for making toners less particulate after fixing to improve light transmission properties, a method is used in which toner particles are buried in the surface layer by the action of the heat and pressure at the time of fixing, as disclosed in Japanese Patent Applications Laid-open No. 2-263642 and No. 7-199515. In these light-transmitting recording materials, the toner is made less particulate after fixing as an effect attributable to the resin constituting the surface layer, and hence they are improved in light transmission properties to have superior projection performance on OHPs. When, however, there is used a resin that can not be well plasticized by the action of the heat and pressure at the time of fixing, the toner particles enter in the surface layer in a very small quantity, so that projected images may have a grayish tone.
The fixing devices described above are all of the type that a release agent such as oil is applied to the fixing roller so that it can be put into service when toner images are fixed. That is, the OHP sheets referred to in the above take no account of an oil-less fixing process, which uses a toner containing wax as a release agent and requires no application of the release agent such as oil to the surface of the fixing roller. Hence, especially when toner images having a small quantity of toner, having an image area percentage of about 5%, are heat-fixed on the above OHP sheets using the toner described above, the toner shows good anti-offset properties at toner image areas because the wax contained in the toner acts as the release agent. However, the wax does not well act as the release agent at areas where the toner images are not formed in a wide range, and hence a phenomenon tends to occur in which the surface layer formed of the thermoplastic resin sticks to the fixing roller. Thus, the recording materials are sought to be improved so as to be suited for the oil-less fixing process making use of the above toner.
Japanese Patent Application Laid-open No. 5-181300 discloses that a toner containing a wax component is heat-fixed to a transparent recording material by the oil-less fixing process in which the release agent such as oil is not applied to the surface of the fixing roller. This publication, however, does not teach at all the fixing of the toner images having a small quantity of toner, having an image area percentage of about 5%.